Oxidative Cross-Coupling of Two Different Phenols: An Efficient Route

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Oxidative Cross-Coupling of Two Different Phenols: An Efficient Route to Unsymmetrical Biphenols Nagnath Yadav More and Masilamani Jeganmohan* Department of Chemistry, Indian Institute of Science Education and Research, Pune 411021, India S Supporting Information *

ABSTRACT: An efficient synthesis of unsymmetrical biphenols via the oxidative cross-coupling of two different phenols in the presence of K2S2O8 and Bu4N+·HSO3− (10 mol %) in CF3COOH at ambient conditions is described. 1:1 Cross-coupling of substituted phenols with naphthols and 1:2 cross-coupling of naphthols with phenol are also disclosed. By using Bu4N+·HSO3−, the homocoupling of phenols or naphthols was controlled. In these reactions, the ortho C−H bond of two different phenols and the ortho and para C−H bond of phenols were coupled together.

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reaction. The metal oxy radical was generated by the chromium catalyst, and hydroxyl or alkoxyl radicals were generated by BDP electrode. A sulfate anion radical (SO4−•) is efficiently used as a potential oxidant for the degradation of environmental pollutant.9a Due to the strong electron-accepting ability of SO4−•, it has been efficiently used to destroy soil carbon pollutants. Although the oxidation potential of SO4−• is 2.6 V, the reactivity between organic compounds is considered to be slow but very selective.9b,c K2S2O8, Na2S2O8, and (NH4)2S2O8 are commonly used sources to generate SO4−•. Interestingly, the sulfate anion radical can be generated very easily at rt without having any external reagents or sources. Our continuous interest in the SO4−• anion radical chemistry prompted us to explore the possibility of using SO4−• as an oxidant for the cross-coupling of two different phenols.10a,b Herein, we report an efficient synthesis of unsymmetrical biphenols via the oxidative crosscoupling of two different phenols in the presence of K2S2O8 in CF3COOH at ambient temperature under air. The crosscoupling reaction was also successfully extended into 1:1 crosscoupling of substituted phenols with naphthols and 1:2 crosscoupling of naphthols with phenol. To accomplish the cross-coupling reaction, initially, a combination of phenol and 2-naphthol substrates such as 4methoxyphenol (1a) and 2-naphthol (2a) were selected (Scheme 1). The cross-coupling reaction of 1a with 2a was tested in the presence of K2S2O8 in CF3COOH at rt for 18 h under air. In the reaction, the homocoupling of 1a (product 3aa in 23% yield), the homocoupling of 2a (product 4aa in 27% yield), and the expected cross-coupling product 5aa in 21% yield were observed. In the reaction, K2S2O8 generates the SO4−• anion radical at ambient temperature. It is believed that the SO4−• anion radical was stabilized by CF3COOH solvent via

he development of a highly efficient, easily accessible, and environmentally friendly method for synthesizing biphenol molecules under mild reaction conditions in a highly atom economical manner is highly important in organic synthesis. Biphenol units are present in various natural products, drug molecules, and functional materials.1 In addition, biphenol molecules are efficiently used as ligands in various organic transformations including enantioselective reactions.2 Generally, nature prefers to synthesize phenolic ligands, biomaterials, isoquinoline alkaloids, and natural products via an oxidative phenol coupling in the presence of oxidative enzymes such as laccase, peroxidase, and cytochrome P450 (CYP) as catalysts.3 Meanwhile, it is also believed that this coupling reaction is involved in the formation of humic materials in soils via enzymatic and abiotic routes.4 Thus, to understand the reaction mechanism and to develop a practical route to synthesize biphenols has been a long time interest among organic chemists.5−7 Symmetrical biphenols are prepared by the oxidative homocoupling of phenols in the presence of a stoichiometric amount of metal salts or metal catalysts along with a stoichiometric amount of oxidants or organic reagents such as hypervalent iodine(III) reagents, DDQ, NaOCl, and molecular oxygen as a terminal oxidant catalyzed by NaNO2.5−7 Although the oxidative coupling of phenols has been known in the literature for several decades, the cross-coupling of two different phenols is not well studied and very challenging due to the formation of many competitive side products. While designing this type of oxidative coupling reaction, control of competitive side products such as homocoupling of phenols, Pummerer’s ketones, C−O bond formation between two phenols, quinones, polymers, and dehydrotrimers is very important.8a Very recently, the cross-coupling of phenols has been successfully achieved by using a chromium salen catalyst or an electrochemical oxidation pathway.8a−c In the reaction, metal oxy or hydroxy or alkoxyl radicals are key intermediates, which abstract hydrogen from phenol and initiates the coupling © XXXX American Chemical Society

Received: May 6, 2015

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DOI: 10.1021/acs.orglett.5b01324 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

In the presence of Bu4N+·HSO3− (10 mol %), 2-naphthol (2a) underwent cross-coupling with 4-methyl-2-methoxy phenol (1b) or 2,6-dimethoxy phenol (1c), providing products 5ba and 5ca in 61% and 82% yields, respectively (Table 1, entries 1 and 2). In

Scheme 1. Cross-Coupling of Phenol and Naphthol

Table 1. Cross-Coupling of Phenols and Naphtholsa

hydrogen bonding. Later, the SO4−• anion radical abstracts hydrogen from 1a or 2a in the presence of CF3COOH, providing a cationic phenol or naphthol radical intermediate 6 and nontoxic HSO4− salt. To gain further insight into the radical formation, the oxidation potential of 1a and 2a was tested by using cyclic voltammetry (see Supporting Information). The oxidation potential of 1a is SHE 0.66 V, and that of 2a is SHE 0.88 V. It is expected that the SO4−• anion radical prefers to abstract hydrogen from 1a, forming a cationic intermediate 6a compared to 2a due to the lower oxidation potential value. To support the formation of a cationic intermediate 6a, a UV−vis spectroscopy study was done. An intense absorption band was observed in the visible region between 400 to 500 nm during the reaction of phenol 1a with K2S2O8 at rt. This study strongly supports that a cationic intermediate is formed during the reaction as previously observed by Kochi’s group.10c−e Later, the nucleophilic addition of 2a into cationic intermediate 6a provides the cross-coupling product 5aa. It is clear that the coupling reaction proceeds via a cationic radical intermediate. We expected that if the reaction is done in the presence of an ionic salt, a negatively charged counterion could coordinate with a cationic phenol radical intermediate 6a and stabilize it. Thus, the possibility is there to increase the yield of cross-coupling product 5aa. With this idea, the reaction was tested with a catalytic amount of 10 mol % of salts such as Bu4N+· I−, Bu4N+·Br−, Bu4N+·Cl−, Bu4N+·F−, and Bu4N+·HSO3−. Bu4N+·I− provided homocoupling product 4aa in 33% yield and cross-coupling product 5aa in 23% yield, respectively. The other homocoupling product 3aa was not observed (Scheme 1). Surprisingly, Bu4N+·HSO3− afforded cross-coupling product 5aa in 61% yield and homocoupling product 4aa in a very minor 1% yield (for mass balance, see Supporting Information). Other salts were not active for the reaction. The same reaction was also done under a nitrogen atmosphere. In the reaction, product 5aa was observed in 60% yield as in the case of under air. This result clearly reveals that the reaction is not air sensitive. K2S2O8 is crucial for the reaction, and without that the reaction did not proceed. Meanwhile, 2.0 equiv of K2S2O8 are also needed to raise the yield of cross-coupling product 5aa. The reaction was also examined with 1.0 and 1.5 equiv of K2S2O8. In these reactions, product 5aa was observed only in 40% and 57% yields, respectively. The cross-coupling reaction was also examined with various solvents such as AcOH, MeOH, ClCH2Cl, tert-amyl alcohol, tert-BuOH, and CF3SO3H. In acetic acid solvent, product 5aa was observed in very low 5% yield and the remaining solvents were not effective. The reaction was also tried with Na2S2O8 and (NH4)2S2O8. (NH4)2S2O8 was also equally effective, providing product 5aa in 60% yield. But, Na2S2O8 provided only product 5aa in 40% yield. It is expected that the solubility of Na2S2O8 is less in organic solvent.

a

All reactions were carried out using 1a−c (1.0 equiv), 2a−c (1.5 equiv), Bu4N+·HSO3− (10 mol %), and K2S2O8 (2.0 equiv) in CF3COOH at ambient conditions for 18 h. bIsolated yield.

the reaction of 1c with 2a, the para C−H bond of 1c was coupled with the α C−H bond of 2-naphthol (2a). The oxidation potential of 1b is 0.7 V, and that of 1c is 0.62 V. Thus, in these reactions, only phenols 1b−c form the cationic radical species, and 2a acts as a nucleophile. Similarly, naphthalene-2,3-diol (2b) reacted with 1a or 1c, affording the cross-coupling products 5ab and 5cb in 64% and 51% yields, respectively (entries 3 and 4). Further, naphthalene-2,7-diol (2c) reacted with 1a or 1c, providing coupling products 5ac and 5cc in 73% and 69% yields, respectively (entries 5 and 6). The mass balance of all these reactions was mentioned in the Supporting Information. In the naphthols 2b and 2d, two OH groups are present on the aromatic moiety. We tried to incorporate two phenol moieties at the α C−H bond of the corresponding OH groups (Scheme 2). Thus, treatment of 2b or 2d with an excess amount of 2,6dimethoxyphenol (1c) (2.2 equiv) gave polycyclic naphthol derivatives 7a and 7b in 71% and 75% yields, respectively (for mass balance, see Supporting Information). Encouraged by the 1:1 and 2:1 cross-coupling of phenols with naphthols, the possibility of cross-coupling of two different phenols was tested (Scheme 3). The cross-coupling of two B

DOI: 10.1021/acs.orglett.5b01324 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Table 2. Cross-Coupling of Two Different Phenolsa

Scheme 2. bis-Arylation of Naphthols 2b and 2d

Scheme 3. Cross-Coupling of Two Different Phenols

different phenols is difficult due to the similar oxidation potential values. Initially, 2-methoxy-4-methylphenol (1b) with 3,4dimethylphenol (1d) were taken as model substrates for the reaction. The oxidation potential of 1b is 0.7 V, and that of 1d is 0.87 V. Thus, 1b prefers to form a cationic radical intermediate in the presence of SO4−•. The oxidation potential difference between 1b and 1d is very small; thus, homocoupling of these phenols would be expected more compared with the crosscoupling product. Treatment of 1b (1.0 equiv) with 1d (1.0 equiv) in the presence of Bu4N+·HSO3− (10 mol %) in CF3COOH at ambient temperature gave the cross-coupling product 8bd in 35% yield along with the homocoupling product of 1b, product 3bb, in 8% yield. Surprisingly, the homocoupling product of 1d was not observed, and the remaining starting material was recovered. To increase the yield of the crosscoupling product, the amount of 1d was increased to 2.5 equiv to 3.0 equiv. Surprisingly, with 3.0 equiv, cross-coupling product 8bd was observed in 53% yield, and no homocoupling product of 1b was detected. In 2.5 equiv, product 8bd was observed in 49% yield. Next, the scope of the cross-coupling reaction was examined with various substituted phenols 1e−n (Table 2). Trisubstituted phenols such as 2,3-dihydro-1H-inden-5-ol (1e), 2,4-dimethylphenol (1f), or 4-tert-butyl-2-methyl phenol (1g) reacted efficiently with 1b, giving the corresponding biphenols 8be−bg in 56%, 57%, and 43% yields (entries 1−3). In the reaction of 1b with 1f, the homocoupling product of 1f was observed in 7% yield. Next, the cross-coupling reaction was examined with disubstituted phenols. Thus, treatment of 2-methyl phenol (1h) with 1b gave a mixture of two regioisomeric products, 2,4′biphenol 8bh in 46% yield and 2,2′-biphenol 8bh′ in 8% yield (eq 1), respectively (entry 4). Interestingly, in the reaction of 2-

a

All reactions were carried out using 1b (1.0 equiv), 1e−n (3.0 mmol), Bu4N+·HSO3− (10 mol %), and K2S2O8 (2.0 equiv) in CF3COOH (0.5 mL) at rt for 18 h. bIsolated yield. cHomocoupling of 1f was observed in 7% yield. dOther regioisomer was observed.

8% and 6% yields, respectively (eq 1). 1,4-Disubstituted phenols such as 4-methyl (1l) or 4-tert-butyl phenols (1m) were also involved in the reaction, giving the corresponding cross-coupling products 8bl and 8bm in 42% and 38% yields, respectively (entries 8 and 9). We have tried the cross-coupling of a less reactive unsubstituted phenol (1n) with 1b. In the reaction, interestingly, 2,2′-biphenol (8an) was observed in 33% yield and the other regioisomeric 2,4′-biphenol was not observed (entry 10). In entry 5, ortho-para product 8bi and, in entry 10, orthoortho product 8bn were observed. The exact reason for the selectivity is not clear; it is possible that the para carbon electron density of 1i and the ortho carbon electron density of 1n could be more and favorable for the coupling reaction. Treatment of 1b or 1d or 1f with 2,6-dimethoxy phenol (1c) under similar reaction conditions gave ortho-para cross-coupling products 8cb−8cf in 48%, 59%, and 56% yields, respectively (Scheme 4). In most phenol cross-coupling reactions, only good to moderate yields were observed. It is important to note that, for the first time in the literature, this type of cross-coupling with various new phenols was demonstrated.

hydroxy phenol (1i) with 1b, only 2,4′-biphenol 8bi was observed in 38% yield (entry 5). In the reaction of 1b with 1i, the ortho C−H bond of 1b was coupled with the para C−H bond of 1i. 1,3-Disubstituted phenols such as 3-methoxy (1j) or 3-methyl (1k) phenols reacted with 1b, affording 2,2′-biphenols 8bj and 8bk in 26% and 49% yields, respectively (entries 6 and 7). In addition, the other regioisomers 8bj′ and 8bk′ were observed in C

DOI: 10.1021/acs.orglett.5b01324 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 4. ortho-para Cross-Coupling of Phenols

G.; Mortimer, A. J.; Keller, P. A.; Gresser, M. J.; Garner, J.; Breuning, M. Angew. Chem., Int. Ed. 2005, 44, 5384. (d) McGlacken, G. P.; Bateman, L. M. Chem. Soc. Rev. 2009, 38, 2447. (e) Allen, S. E.; Walvoord, R. R.; Padilla-Salians, R.; Kozlowski, M. C. Chem. Rev. 2013, 113, 6234. (2) Chen, Y.; Yekta, S.; Yudin, A. K. Chem. Rev. 2003, 103, 3155. (3) (a) Aldemir, H.; Richarz, R.; Gulder, T. A. M. Angew. Chem., Int. Ed. 2014, 53, 8286. (b) Dagne, E.; Steglich, W. Phytochemistry 1984, 23, 1729. (c) Zenk, M. H.; Geeardy, R.; Stadler, R. J. Chem. Soc. Chem. Commun. 1989, 1725. (d) Gerardy, R.; Zenk, M. H. Phytochemistry 1992, 32, 79. (e) Ikezawa, N.; Iwasa, K.; Sato, F. J. Biol. Chem. 2008, 283, 8810. (f) Kraus, P. F. X.; Kutchan, T. M. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 2017. (g) Davin, L. B.; Wang, H. B.; Crowell, A. L.; Bedgar, D. L.; Martin, D. M.; Sarkanen, S.; Lewis, N. G. Science 1997, 275, 362. (h) Freudenberg, K. Nature. 1959, 183, 1152. (4) Pal, S.; Bollag, J. M.; Huang, P. M. Soil Biol, Biochem. 1994, 26, 813. (5) Selected papers: (a) Bringmann, G.; Gulder, T.; Gulder, T. M.; Breuning, M. Chem. Rev. 2011, 111, 563. (b) Grzybowski, M.; Skonieczny, K.; Butenschon, H.; Gryko, D. T. Angew. Chem., Int. Ed. 2013, 52, 9900. (c) Doussot, J.; Guy, A.; Ferroud, C. Tetrahedron Lett. 2002, 41, 2545. (d) Ding, K.; Wang, Y.; Zhang, L.; Wu, Y. Tetrahedron 1996, 52, 1005. (e) Yamamoto, K.; Fukushima, H.; Okomoto, Y.; Hatada, K.; Nakazaki, M. J. Chem. Soc., Chem. Commun. 1981, 46, 2547. (f) Dewar, M. J. S.; Nakaya, T. J. Am. Chem. Soc. 1968, 90, 7134. (6) Selected papers: (a) Wendllandt, A. E.; Suess, A. M.; Stahl, S. S. Angew. Chem., Int. Ed. 2011, 50, 11062. (b) Matsushita, M.; Kamata, K.; Yamaguchi, K.; Mizuno, N. J. Am. Chem. Soc. 2005, 127, 6632. (c) Hay, A. S.; blanchard, H. S.; Endres, G. F.; Eustance. J. Am. Chem. Soc. 1959, 81, 6335. (d) Jiang, Q.; Sheng, W.; Tian, M.; Tang, J.; Guo, C. Eur. J. Org. Chem. 2013, 1861. (e) Wallis, P. J.; Booth, K. J.; Patti, A. F.; Scott, J. L. Green Chem. 2006, 8, 333. (f) Esguerra, K. V. N.; Fall, Y.; Petitjean, L.; Lumb, J.-P. J. Am. Chem. Soc. 2014, 136, 7662. (g) Hwang, D.-R.; Chen, C.-P.; Uang, B.-J. Chem. Commun. 1999, 1207. (7) Selected papers: (a) Ashenhurst, J. A. Chem. Soc. Rev. 2010, 39, 540. (b) Su, B.; Li, L.; Hu, Y.; Liu, Y.; Wang, Q. Adv. Synth. Catal. 2012, 354, 383. (c) Dohi, T.; Maruyama, A.; Yoshimura, M.; Morimoto, K.; Tohma, H.; Kita, Y. Angew. Chem., Int. Ed. 2005, 44, 6193. (d) Kirste, A.; Schnakenburg, G.; Waldvogel, S. R. Org. Lett. 2011, 13, 3126. (e) Mirk, D.; Willner, A.; Frohlich, R.; Waldvogel, S. R. Adv. Synth. Catal. 2004, 346, 675. (f) Zhai, L.; Shukla, R.; Wadumethrige, S. H.; Rathore, R. J. Org. Chem. 2010, 75, 4748. (g) Morimoto, K.; Sakamoto, K.; Ohnishi, Y.; Miyamoto, T.; Ito, M.; Dohi, T.; Kita, Y. Chem.Eur. J. 2013, 19, 8726. (h) Malkowsky, I. M.; Griesbach, U.; Putter, H.; Waldvogel, S. R. Eur. J. Org. Chem. 2006, 4569. (8) (a) Lee, Y. E.; Cao, T.; Torruellas, C.; Kozlowski, M. C. J. Am. Chem. Soc. 2014, 136, 6782. (b) Elsler, B.; Schollmeyer, B.; Dyballa, K. M.; Franke, R.; Waldvogel, S. R. Angew. Chem., Int. Ed. 2014, 53, 5210. (c) Kirste, A.; Elsler, B.; Schnakenburg, G.; Waldvogel, S. R. J. Am. Chem. Soc. 2012, 134, 3571. (9) (a) Ahmad, M. Persulfate activation by major soil minerals. Ph.D. Thesis, Department of Civil and Environmental Engineering, Washington State University, August 2009. (b) Buxton, G. E. P.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. 1987, 17, 513. (c) Madhavan, V.; Zemel, H.; Fessenden, R.; Neta, P. J. Am. Chem. Soc. 1977, 99, 163. (10) (a) More, N. Y.; Jeganmohan, M. Org. Lett. 2014, 16, 804. (b) More, N. Y.; Jeganmohan, M. Chem.Eur. J. 2015, 21, 1337. (c) Vasudeva, W. C. J. Chem. Soc., Perkin Trans. 2 1975, 697. (d) Sankararaman, S.; Haney, W. A.; Kochi, J. K. J. Am. Chem. Soc. 1987, 109, 7824. (e) Kita, Y.; Tohma, H.; Hatanaka, K.; Takada, T.; Fujata, S.; Mitoh, S.; Sakurai, H.; Oka, S. J. Am. Chem. Soc. 1994, 116, 3684.

We have checked the oxidation potential value of several phenols and naphthols and compared them with the present reaction. This result clearly reveals that the oxidation potential of phenol up to 0.7 V satisfactorily reacts with SO4−•, providing the corresponding cationic phenolic radical intermediate. If the oxidation potential is above 0.7 V, the substrate does not oxidize and participate in the reaction. For example, 4-tert-butyl-2methyl phenol (1g) (Epox = 0.81) or 2-naphthol (2a) (Epox = 0.81) did not react with 3,4-dimethylphenol (1d) (Epox = 0.87). In these reactions, not even the homocoupling product was observed. However, 1a (Epox = 0.66 V), 1b (Epox = 0.70 V), and 1c (Epox = 0.62 V) were nicely involved in the reaction. In conclusion, we have demonstrated an efficient synthesis of unsymmetrical biphenols via the oxidative cross-coupling of two different phenols in the presence of K2S2O8 and Bu4N+·HSO3− (10 mol %) in CF3COOH at ambient conditions. 1:1 Crosscoupling of substituted phenols with naphthols and 2:1 crosscoupling of substituted phenols with naphthols were also described. In these reactions, the ortho C−H bond of two different phenols and the ortho and para C−H bond of phenols were coupled together. By using Bu4N+·HSO3−, the homocoupling of phenols or naphthols and also overoxidation of the desired cross-coupling products were controlled.

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ASSOCIATED CONTENT

S Supporting Information *

General experimental procedure and characterization details. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.5b01324.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the CSIR (02(0179)/14/EMR-II), India for the support of this research. N.Y.M. thanks the CSIR for a fellowship. We thank Dr. K. Krishnamoorthy (NCL Pune) for checking the oxidation potential value of phenols.

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REFERENCES

(1) Selected reviews: (a) Hassan, J.; Sévignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M. Chem. Rev. 2002, 102, 1359. (b) Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Angew. Chem., Int. Ed. 2005, 44, 4442. (c) Bringmann, D

DOI: 10.1021/acs.orglett.5b01324 Org. Lett. XXXX, XXX, XXX−XXX